A magnetic field consisting of a permanent magnet is formed at the electromagnetic force generating portion of an electronic balance, and a winding (force coil) is arranged with respect to the space portion formed in the magnetic field so as to intersect the magnetic field. The load of a weighed object is transmitted to the electromagnetic force generating portion through a lever mechanism and attempts to displace the force coil of the lever mechanism formed integrally and arranged in the electromagnetic portion. The configuration is such that with respect thereto the displacement of the force coil is brought into equilibrium by Lorentz force generated by passing currents through the force coil, and the load of the weighed object is calculated from the amount of electricity used at that time.
FIG. 7 of the present application schematically shows the load measuring mechanism in the above-described electronic balance. In this mechanism, a Roberval's mechanism is constituted by upper and lower sub-levers 50, 51, a displacement member (hereinafter referred to as “floating frame”) 52 which is displaced by the load of a weighed object imposed on a weighing pan 56, and a stationary block 58 fixed to the main body of the weighing apparatus; a lever member 54 is constituted with respect to this Roberval's mechanism through a suspending band 53 which is a connecting member, so as to be rockable by means of a fulcrum 55.
In the above-described constitution, the load W of a weighed object imposed on the weighing pan 56 is guided by this Roberval's mechanism to be transmitted to the lever member 54, and the lever member 54 attempts to be displaced in the X direction by the load W. With respect thereto, electric power is supplied to a force coil 60 arranged close to a permanent magnet 57 in the electromagnetic portion so as to generate Lorentz force for bringing the lever member 54 into equilibrium, and the amount of this electric power is converted to the load of the weighed object thereby to measure the load W of the weighed object.
Here, the constitution of FIG. 7 only schematically shows the principle of the load measuring mechanism portion of an electronic balance; in a real machine, the load measuring mechanism portion has a considerably complex structure, having a double lever structure in order to have a greater lever ratio, or having various electronic circuits provided in combination, for example.
On the other hand, need for reduction in the size of the whole weighing apparatus is expected to be further more severe in the future. As a solution to that, a load measuring mechanism is becoming necessary which has a lever mechanism of a high lever ratio so as to measure a relatively large load by an electromagnetic portion of small capacity. In this case, since the setting of the lever ratio by one lever member is limited, a method of greatly increasing the lever ratio by connecting a plurality of lever members is adopted.
A method of assembling those constituted as separate parts by spring materials, aluminum materials or the like is usually adopted for the load measuring mechanism shown in FIG. 7. However, reduction in the size of the mechanism by such an assembly method is limited. Moreover, it requires labor and time, and further a high degree of experience; for example, instrumental error is generated depending on the degree of screwing or the like at the time of assembly, or fine adjustment is performed after assembly.
From the above-described viewpoint, a constitution is proposed in which the load measuring mechanism is integrally formed in advance so that the assembly process of this mechanism can be substantially omitted. In this constitution, for example, a metal block of aluminum or the like is worked to form the Roberval portion, lever portion and the like being connected; portions necessary for a load measuring mechanism are formed from one metal block.
While the above-described problems in the assembly process can be avoided by the constitution as described above, other problems as follows arise.
To constitute an integral type mechanism from a metal block, highly accurate and complex working processing such as wire cutting and subtle cutting working needs to be performed. Consequently, cost for working the parts greatly increases as compared with the case of the conventional assembling type in which separate parts are manufactured. Further, if working of part of the worked portions in the metal block is inappropriate, since it is an integral type, the whole block is unusable; thus the yield of the product is poor, which is also an important cause of increase in the price. In the same way, if a malfunction occurs in the load measuring mechanism at the stage of using the electronic balance, since replacement of parts is impossible, the whole of the integral type load measuring mechanism has to be replaced so that repair cost is inevitably high.
From the above-described viewpoints, a completely integral type load measuring mechanism as described above has many problems with respect to yields of products, maintenance and the like at the stage of being constituted as a real machine.
In consideration of this point, conventionally some proposals have been made to achieve both easy assembly of the mechanism and reduction of manufacturing cost of the product by integrating only part of the load measuring mechanism.
In a device shown in Japanese Utility Model Application Laid-Open No. S64-5127 which is a prior art document, thin plates in which a fulcrum, a lever portion and the like are formed by machining working in advance are screw-fastened through spacers, thereby to constitute as a whole a load measuring mechanism having the Roberval's mechanism and the lever mechanism. By thus making a constitution from a plurality of parts, any of the parts being defective can be replaced so that the yields of the product are expected to be considerably improved as compared with the completely integral type constitution.
In the device of the above-described prior art document, two thin plates of the same shape having a fulcrum or a lever portion formed by cutting are screw-fastened to both sides of spacers to constitute as a whole a Roberval's mechanism and a lever mechanism. Therefore, even a slight error in the state of attachment of the two thin plates can generate distortion in the whole of the Roberval's mechanism and lever mechanism to cause trouble in load measuring. Therefore, the assembly requires precise care. In addition, this device has a constitution in which the Roberval's mechanism receiving the real load in the vertical direction is fixed by fastening screws arranged in the horizontal direction; therefore there is a possibility that the position of attachment of the members is changed by the load and stable performance as a measuring apparatus cannot be provided.
In the invention described in Japanese Patent Application Laid-Open No. 2002-148105, the Roberval's mechanism and part of the lever mechanism are constituted integrally from a metal block, and a long beam is connected to the lever portion of the metal block to set a high lever ratio; it is a completely integral type except for the beam portion and has problems common to the above-described completely integral type constitution. In the same way, Japanese Patent No. 2570405 has the major portion of an integral type and also has problems common to the completely integral type constitution.
The invention described in Japanese Patent Application Laid-Open No. 2001-066178 has an asymmetric constitution in which a second lever in a double lever constitution is arranged at one side of the Roberval's mechanism; it is not necessary to perform fine adjustment so as to completely match the attachment of two members as in a lever portion consisting of two thin plates of the same shape; however, there is a problem that since the second lever is positioned projecting at a side portion of the Roberval's mechanism portion in the constitution, need for reduction in the size of the mechanism as well as reduction of assembly error and cost reduction by means of reduction of the number of components cannot be sufficiently met.
In addition, Japanese Patent Application Laid-Open No. 2000-283829, the object of which is to overcome the poor yields of the completely integral type mechanism and provide a compact and high performance load transmitting mechanism equivalent to the completely integral type mechanism, describes a structure in an electromagnetic balancing-type balance having a Roberval's mechanism and a load transmitting beam for transmitting an imposed load to an electromagnetic portion by being guided by the Roberval's mechanism, comprising: an integral type Roberval's mechanism portion in which a portion operating as a Roberval is formed by a space portion being formed between a load receiving portion and a stationary portion; a load transmitting beam; and a member to be the fulcrum and a member to be the force point of the load transmitting beam, wherein attachment portions of the members constituting the fulcrum and force point are formed in the space portion such that the fulcrum and force point of the load transmitting beam are positioned in the space portion. However, it has a problem that it cannot sufficiently satisfy needs such as easy assembly of the mechanism and minimization of fine adjustment after assembly.
Other related prior art documents include Japanese Patent Application Laid-Open No. 2002-148105, U.S. Pat. Nos. 4,799,561, 5,962,818, 6,472,618 B1, 6,787,714 B2, 6,861,593 B2, European Application No. EP1189043 A1, Germany Patent No. DE19804439 C1, Germany Patent No. DE1034272 B3, and Germany Patent No. DE10332400 B3, but none of the inventions described in these documents solves all of the technical problems described above.